The products, particularly steel slabs, which are heated before rolling in furnaces, are supported on cross-bars or supports cooled by circulation of water or vapour, via supports called studs or lugs.
These studs are generally cylindrical in form and placed on the support tubes at regular intervals and maintained on the tube by welding. Part of the height of the studs is coated by the insulating refractory lining which surrounds the support.
The purpose of the studs is to separate the slab from the support in order to protect the lining from the mechanical shocks which may occur during the movements ensuring advance of the products in the furnace.
The fact of separating the slab from the support also leads to the decrease in the shadow effect of the support on the slab. In fact, the heat transmitted to the slab in the furnace is essentially transmitted by radiation and the proximity of a cooled support, even insulated by a refractory lining, forms an obstacle to the radiation of the furnace on the product. This results in a lower-temperature band on the product, which represents the mark of the support, often known as black mark.
The gap existing between the support and the slab is determined by the height of the stud. It is difficult to increase this height a great deal.
In fact, the part of the stud which emerges from the refractory lining receives the radiation of the furnace and its temperature increases rapidly with height.
For reasons of mechanical strength (resistance to crushing, creeping), it is necessary that the temperature of the stud does not go beyond a certain value, which value is a function of the load supported by the stud when it is in contact with the slab. In fact, it is observed that, if a stud has, from the beginning, a height greater than the usual height, it is rapidly crushed until this value is reached.
It is not possible, on the other hand, to extend the refractory lining of the support up to the upper part of the stud, so as to heat-insulate said stud. In fact, the products, during heating, deposit carbon, due to oxidation and this carbon accumulates on the refractory lining. These must therefore be a sufficient space between the slab and the lining to allow the formation of a slope of carbon which is then maintained at constant height. If this space is insufficient, the carbon transmits to the lining the weight of the slab, which results in destruction of the refractory lining.